Efficacy and hair deposition of pyrethroid- and organophosphate-based acaricides applied at different working pressures for the control of Rhipicephalus microplus in dairy cattle

Rhipicephalus microplus significantly affects dairy cattle, with spray-applied acaricides commonly used for its control. However, increasing resistance and variations in application methods may compromise treatment efficacy. This study evaluated the efficacy of acaricide formulation containing pyrethroids and organophosphates, as well as its deposition in bovine hair when applied at different working pressures. Cattle were divided into three groups (n = 10 each): Group 1 received treatment at 50 psi using a backpack sprayer, Group 2 at 300 psi using a power sprayer, and Group 3 served as an untreated control. Female ticks (4.5-8 mm) were counted on days 3, 7, 14, 21, and 35 post-treatment (DPT). Hair samples were collected from six body regions (dewlap, belly, between the legs, loin, legs, and neck) at 0, 1, 2, and 3 DPT. Residual concentrations of cypermethrin, chlorpyrifos, and fenthion were measured using direct analysis in real-time mass spectrometry (DART-MS). A significant difference (p ≤ 0.05) in tick reduction was observed between the treated groups only at 3 DPT. Peak efficacy occurred on day 21 in both treated groups, with efficacy ranging from 56.7% to 97.5% (50 psi) and 78.9-87.4% (300 psi). Cypermethrin levels were consistently higher at 50 psi across all days and most body regions. Chlorpyrifos and fenthion showed higher concentrations on day 0 at 300 psi, especially in the loin and neck. The pressure did not lead to significant differences in overall efficacy against R. microplus. However, it significantly affected the deposition of active ingredients in bovine hair.